The background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
While almost all of the lead from lead acid batteries is recycled, most known processes are environmentally and economically problematic. For example, where lead is recycled using smelting operations, air and water pollution along with production of substantial quantities of toxic waste have lead to the closure of many recycling plants. Moreover, to meet the stringent demands on emissions and energy efficiency, lead acid battery recycling has forced operations to ever increasing throughput, leading to logistics challenges.
To help overcome some of the difficulties with smelting operations, various systems and methods for lead recovery without smelting have been developed. For example, U.S. Pat. No. 4,460,442 teaches a lead recovery process in which lead and lead dioxide are ground and reacted with a strong alkaline solution to produce solid minium (Pb3O4) that is then subjected to further reaction with hot fluorosilic or fluoroboric acid to dissolve the lead, which is then electroplated from these acids onto a graphite anode. Similarly, U.S. Pat. No. 4,769,116 teaches carbonation reactions of lead paste and subsequent reaction with fluorosilic or fluoroboric acid to form an electrolyte from which lead is plated. All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply. While such process advantageously avoids smelting, various difficulties nevertheless remain. Most notably, digestion with fluorosilic or fluoroboric acid is environmentally undesirable and the residual materials contain substantial quantities of lead sulfate.
Lead paste can also be desulfurized using caustic soda (NaOH) or soda ash (Na2CO3) to produce from lead sulfate the corresponding lead hydroxides or lead carbonates. Alternatively, amine solvents can be used to desulfurized lead paste and produce purified lead sulfate and recycled amine solvent as is described elsewhere (Journal of Achievements in Materials and Manufacturing Engineering 2012, Vol. 55(2), pp. 855-859). Unfortunately, such process does allow for production of pure elemental lead.
Desulfurization can be followed by treatment of lead oxides with an acid and a reducing agent to form a lead salt that is then reacted with a second base under a CO2-free atmosphere at an elevated temperature to form PbO as described in WO 2015/057189. While such process allows for production of PbO, multiple solvent treatment steps and reagents are needed, and pure elemental lead is not readily obtained from such process. Similarly, US 2010/043600 discloses a process for the recovery of high purity lead compounds from paste in which lead oxide is first dissolved in an acid, in which insoluble lead dioxide is reduced, and in which the so obtained lead oxide is converted to lead sulfate that can then be converted to the corresponding carbonate, oxide, or hydroxide. Unfortunately, such process is relatively complex and is thus typically economically unattractive.
In yet another example, WO 2015/084950 describes a process in which lead paste from a battery is first reacted with nitric acid to convert lead dioxides to lead nitrate, and in which lead sulfate is recovered from solution using sulfuric acid to so regenerate the nitric acid. Lead sulfate from the battery paste is subjected to alkali to precipitate lead oxides that are then, after removal of sulfate, converted to lead carboxylate as a raw material for lead monoxide. Unfortunately, the processes described in the '950 application are complex and may not always result in complete recycling and production of pure lead. Significant improvements have been disclosed in WO 2015/077227 where lead paste from lead acid batteries is dissolved in a solvent system that allows for digestion of both lead oxide and lead sulfate, and from which elemental lead can be electrolytically deposited in a chemically pure form. While such system advantageously allows for high lead recovery in a conceptually simple and effective manner, sulfate accumulation in the electrolyte will nevertheless require solvent treatment.
Thus, even though there are numerous systems and methods for lead recycling known in the art, there is still a need for improved systems and methods that produce high purity lead in a simple and economically effective manner.